Hydroxyl radical (•OH) is one of reactive oxygen radicals. Hydroxyl radical (•OH) has extremely strong oxidizing property with an oxidation reduction potential of 2.80 V, which is second only to that of fluorine atom and is capable of reacting with most inorganics or organics at a diffusion controlled rate with a reaction rate constant of generally greater than 108 mol L−1 s−1. In the field of environmental science, hydroxyl radicals are used for degradation treatment of organic pollutants and are the most important reactive intermediates in the advanced oxidation processes (AOPs) for sewage treatment.
There are many methods for generating hydroxyl radicals, which can be broadly classified into chemical catalysis method, ozone/hydrogen peroxide photolysis method, photocatalysis method, electrocatalysis method, ray method, and so on. The chemical catalysis method generally uses Fenton reaction, and hydroxyl radicals are generated by catalyzing decomposition of hydrogen peroxide with iron ions. Although the method is simple, easy and cheaper, a large amount of iron-containing sludge is generated when it is applied on a larger scale, causing inconvenient subsequent processing. Ozone and hydrogen peroxide are photolyzed under the irradiation of ultraviolet light and hydroxyl radicals can be generated; however, it is needed to add hydroxyl radical precursors, such as ozone and hydrogen peroxide, and there are more side reactions. Generating hydroxyl radicals by photocatalysis with semiconductor titanium dioxide particles and the like as a catalyst needs to ensure that the catalyst is in a suspended state, and it is also needed to separate the photocatalyst simultaneously for the photocatalysis, providing poor continuous operation, and the dissolved oxygen has a greater influence on the generation of hydroxyl radicals by photocatalysis using titanium dioxide. The method for generating hydroxyl radicals by electrocatalysis has higher requirements for the dissolved oxygen in water and its catalytic components, and has lower current efficiency. There are problems of higher cost and greater harm to the human body in the ray method.
Therefore, the current commonly used methods for generating hydroxyl radicals have more side reactions, have poor operability and low efficiency, or have a great harm to the environment or the human body. Accordingly, the existing methods for generating hydroxyl radicals each have their own problems, and thus it is difficult for their popularization and application.